Pattern generator with improved precision

ABSTRACT

The present invention relates to a method for creating a pattern on a workpiece with better pattern fidelity and dimensional precision. The method comprises the steps of: providing a source for emitting electromagnetic radiation, illuminating by said radiation a spatial light modulator (SLM) having a multitude of pixels, projecting an image of the modulator on the workpiece, moving said workpiece and/or projection system relative to each other, reading from an information storage device a digital description of the pattern to be written, extracting from the pattern description a sequence of partial patterns, converting said partial patterns to modulator signals, and feeding said signals to the modulator, further coordinating the workpiece, the modulator and the radiation source, so that said pattern is stitched together from the partial images, further exposing said pattern in at least two separate exposures, where at one exposure corrections are applied for errors occurring during another exposure.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SE99/00291 which has an Internationalfiling date of Mar. 2, 1999, which designated the United States ofAmerica.

FIELD OF THE INVENTION

The present invention relates to printing of patterns with extremelyhigh precision on photosensitive surfaces, such as photomasks forsemiconductor devices and displays. It also relates to direct writing ofsemiconductor device patterns, display panels, integrated opticaldevices and electronic interconnect structures. Furthermore, it can haveapplications to other types of precision printing such as securityprinting. The term printing should be understood in a broad sense,meaning exposure of photoresist and photographic emulsion, but also theaction of light on other light sensitive media such as dry-processpaper, by ablation or chemical processes activated by light or heat.Light is not limited to mean visible light, but a wide range ofwavelengths from infrared (IR) to extreme UV. Of special importance isthe ultraviolet range from 370 nm (UV) through deep ultraviolet (DUV),vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) down to a fewnanometers wavelength. EUV is in this application defined as the rangefrom 100 nm and down as far as the radiation is possible to treat aslight. A typical wavelength for EUV is 13 nm. IR is defined as 780 nm upto about 20 μm.

In a different sense the invention relates to the art and science ofspatial light modulators and projection displays and printers using suchmodulators. In particular the invention improves the grey-scaleproperties, the image stability through focus and image uniformity andthe data processing for such modulators by application of analogmodulation technique. The most important use of the analog modulation isto generate an image in a high-contrast material such as photoresistwith an address grid, i.e. the increment by which the position of anedge in the pattern is specified, that is much finer than the gridcreated by the pixels of the spatial light modulator.

BACKGROUND OF THE INVENTION

It is known in the current art to build precision pattern generatorsusing projection of micromirror spatial light modulators (SLMs) of themicromirror type (Nelson, U.S. Pat. No. 5,148,157, 1988, Kuck, EP0610183, 1990). To use an SLM in a pattern generator has a number ofadvantages compared to the more wide-spread method of using scanninglaser spots. The SLM is a massively parallel device and the number ofpixels that can be written per second is extremely high. The opticalsystem is also simpler in the sense that the illumination of the SLM isnon-critical, while in a laser scanner the entire beam path has to bebuilt with high precision. Compared to some types of scanners, inparticular electrooptic and acoustooptic ones, the micromirror SLM canbe used at shorter wavelengths since it is a purely reflective device.

In both references cited above the spatial modulator uses only on-offmodulation at each pixel. The input data is converted to a pixel mapwith one bit depth, i.e. with the values 0 and 1 in each pixel. Theconversion can be done effectively using graphic processors or customlogic with area fill instructions.

In a previous application by the present inventor Sandstrom (Sandstromet al., EP 0567076, 1990), the ability to use an intermediate exposurevalue at the boundary of a pattern element to fine-adjust the positionof the element's edge in the image created by a laser scanner wasdescribed.

It is also known in the art to create a grey-scale image, preferably forprojection display of video images and for printing, with an SLM byvariation of the time a pixel is turned on or by printing the same pixelseveral times with the pixel turned on a varying number of times. Thepresent invention devices a system for direct grey-scale generation witha spatial light modulator, with a special view to the generation ofultra-precision patterns. Important aspects in the preferredembodiments, are uniformity of the image from pixel to pixel andindependence of exact placement of a feature relative to the pixels ofthe SLM and stability when focus is changed, either with intention orinadvertently.

Specifically there are problems with the prior art to achieve adequatepattern fidelity and dimensional precision in pattern generators usingan SLM.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor creating a pattern on a workpiece with better pattern fidelity anddimensional precision.

This object is achieved with a method according to the appended claims.

Specifically the method for creating a pattern according to theinvention comprises the steps of:

providing a source for emitting electromagnetic radiation in thewavelength range from EUV to IR,

illuminating by said radiation a spatial light modulator (SLM) havingmultitude of modulating elements (pixels),

projecting an image of the modulator on the workpiece,

moving said workpiece and/or projection system relative to each other,

reading from an information storage device a digital description of thepattern to be written,

extracting from the pattern description a sequence of partial patterns,

converting said partial patterns to modulator signals, and feeding saidsignals to the modulator,

further coordinating the movement of the workpiece, the feeding of thesignals to the modulator and the intensity of the radiation, so thatsaid pattern is stitched together from the partial images created by thesequence of partial patterns,

further exposing said pattern in at least two separate exposures, whereat one exposure corrections are applied for errors occurring duringanother exposure.

By such an additional exposure, several types of errors could becompensated for.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a printer in prior art. The SLM consists of micromirrorsthat deflect the light from the lens pupil.

FIG. 2 shows a number of pixel designs with the upper four pixels in anoff state and the remaining five pixels turned on.

FIG. 3 shows an array of pixels moving up and down like pistons, therebycreating a phase difference. This is how an edge can be fine-positionedwith a phase-type SLM.

FIG. 4 shows a schematic comparison between an SLM with deflectingmirrors and an SLM with deforming mirrors.

FIG. 5 shows a flow diagram of a method for translating and feeding datato the SLM.

FIG. 6 shows a preferred embodiment of a pattern generator according tothe invention.

FIG. 7 shows schematical intensity curves for explaining the use of anadditional correction exposure.

DESCRIPTION OF THE INVENTION

The basis for understanding the invention is the generic arrangement inFIG. 1 which shows a generic projection printer with an SLM. Spatiallight modulators based on reflection come in two varieties, thedeflection type (Nelson) and the phase type (Kuck). The differencebetween them may in a particular case with micromirrors seen small, butthe phase SLM extinguishes the beam in the specular direction bydestructive interference, while a pixel in a deflection SLM deflects thespecular beam geometrically to one side so that it misses the apertureof the imaging lens as shown in FIG. 1. For ultra-precise printing asperformed in the current invention the phase-modulating system asdescribed by Kuck is superior to the deflecting type. First, it hasbetter contrast since all parts of the surface, also hinges and supportposts, take part in the destructive interference and total extinctioncan be achieved. Second, a system that works by deflecting the light tothe side is difficulty to make symmetrical around the optical axis atintermediate deflection angles, creating a risk of feature instabilitywhen focus is changed. In the preferred embodiments the phase type isused, but if one accepts or designs around the asymmetry of thedeflection type it could also be used. This is illustrated schematicallyin FIG. 4. In the first FIG. 4a, a non-deflected micromirror 401 isilluminated, and the reflected light is not directed towards theaperture 402, and, hence, the light does not reach the substrate 403. InFIG. 4b, on the other hand, the mirror is fully defected, and all thereflected light are directed towards the aperture. In an intermediateposition only part of the reflected light will reach the substrate,which is shown in FIG. 4c. However, in this case the light is notsymmetrical around the optical axis for the lens 404, and there is anoblique incidence on the substrate. Hereby the distance between the lensand the substrate area becomes very critical, and small changes, such asthe one being indicated by the dashed position for area, causessignificant displacements of the features on the substrate. A way tosolve this problem is indicated by the FIGS. 4d-f. Here, a firstexposure is made with a first deflection angle for the micromirror, andthereafter a second exposure, preferably with the same light dose, ismade for a second deflection angle, being complementary to the firstangle. Hereby, the combination of the first and second exposure issymmetrical around the optical axis for the lens. Another way to solvethe problem is to use deforming mirror 401′, such as is shown in FIG.4g, whereby the reflected light is evenly distributed over aperture.This last figure could schematically represent two cases, a phase typeSLM (described below) or a deflection SLM, where light is reflected fromdifferent parts of the mirror.

The phase SLM can be built either with micromachined mirrors, so calledmicromirrors, or with a continuous mirror surface on a supportingsubstrate that is possible to deform using an electronic signal. In Kück1990 a viscoelastic layer controlled by an electrostatic field is used,but it is equally possible, especially for very short wavelengths wheredeformations of the order of a few nanometer are sufficient, to use apiezoelectric solid disk that is deformed by electric field or anotherelectrically, magnetically or thermally controlled reflecting surface.For the remainder of this application an electrostatically controlledmicromirror matrix (one- or two-dimensional) is assumed, although otherarrangements as described above are possible, such as transmissive orreflecting SLMs relying on LCD crystals or electrooptical materials astheir modulation mechanism, or micromechanical SLMs using piezoelectricor electrostrictive actuation.

The invention preferably uses a micromirror where the phase modulationis variable to obtain a variable amount of light reaching the pupil ofthe projection lens. FIG. 2 shows some multi-element mirrors. The tiltsof the various parts of the mirrors are unimportant. In fact one elementby itself would direct the light toward the lens while another woulddirect it outside of the pupil. The correct way to understand thefunction is to look at the complex amplitude reaching the center of thepupil from each infinitesimal area element of the mirror and integratethe amplitude over the mirror. With a suitable shape of the mirror it ispossible to find a deformation where the complex amplitudes add up toalmost zero, corresponding to no light reaching the pupil. This is theoff-state of the micromirror, while a relaxed state where the mirrorsurface is flat and the complex amplitudes add in phase is the on-state.Between the on and off-states the amount of light in the speculardirection is a continuous but non-linear function of the deformation.

The pattern to be written is normally a binary pattern, such as aphotomask pattern in chrome on a glass substrate. In this context binarymeans that there are no intermediate areas: a certain point on thephotomask surface is either dark (covered with chrome) or clear (nochrome). The pattern is exposed in photoresist by the projected imagefrom the SLM and the photoresist is developed. Modern resists have highcontrast, meaning that a small percentage change in exposure makes thedifference between full removal of the resist in the developer andhardly any removal at all. Therefore the photoresist has an edge that isnormally almost perpendicular to the substrate surface, even though theaerial image has a gradual transition between light and dark. The chromeetching does further increase the contrast, so that the resulting imageis perfectly binary: either opaque or clear with no intermediate areas.

The input data is in a digital format describing the geometry of thepattern to be written on the surface. The input data is often given in avery small address unit, e.g. 1 nanometer, while setting pixels in theSLM to either on or off gives a much coarser pattern. If a pixel on theSLM is projected to a 0.1 μm pixel in the image, a line can only have awidth that is an integer number of pixels (n*0.1 μm where n is aninteger). An address grid of 0.1 μm was enough until recently, but theadvent of so called optical proximity Correction OPC makes a grid of 1-5nanometers desirable. In OPC the size of features in the mask aremodified slightly to compensate for predicted optical image errors whenthe mask is used. As an example, when a mask with four parallel lines0.8 μm wide is printed in a modern 4× reduction stepper (a projectionprinter for semiconductor wafers) they will in a typical case print aslines 0.187, 0.200, 0.200, and 0.187 μm wide although they were intendedto have the same width. This can be predicted by simulation of the imageformation and the user of the mask may use OPC to compensate in themask. Therefore he wants the first and last line in the mask to be4*0.213=0.852 μm instead of 0.800 μm. With an address grid of 0.1 μm hecannot make the correction, but with 5 nm address grid or finer suchcorrection are possible.

In FIG. 5 the method for providing data for the SLM is shown in a flowdiagram. The first step, S1, is to divide the pattern data for thepattern to be written into separate pattern fields. The pattern data ispreferably received in digital form. Thereafter, in step S2, the fieldsare rasterised, and thereby assigned different exposure values. Thesevalues are then corrected for nonlinear response, step S3, andpixel-to-pixel variations, step S4. Finally, the pixel values areconverted to drive signals and forwarded to the SLM.

The invention preferably uses intermediate values between the off-stateand on-state to create a fine address grid, e.g. {fraction (1/15)},{fraction (1/25)}, {fraction (1/50)}, of the size of a pixel. A printedfeature consists of pixels in the on state, but along the edge it haspixels set to intermediate values. This is done by driving the pixelswith other voltages than the on and off voltages. Since there areseveral cascaded non-linear effects (the position of the edge versusexposure at the pixels at the boundary, the exposure vs. thedeformation, and the deformation vs. the electric field) a non-lineartransformation from the input data to the electric field is needed.Furthermore this transformation is calibrated empirically at regulartime intervals.

FIG. 3 shows an array of pixels moving up and down like pistons, therebycreating a phase difference. The figure shows how the pixels arecontrolled to create the reflectivity in the inset. The bright areashave pixels with 0 phase, while dark areas are created by pixels withalternating +90 and −90 degree phase. The oblique boundaries betweenbright and dark areas are created by intemediate values of phase. Thisis how an edge can be fine-positioned with a phase-type SLM. However,other types of SLM with intermediate values could be used in the samemanner. The imaging properties with the phase SLM driven intointermediate values are complex and it is far from obvious that the edgewill be moved in FIG. 3. However, it has been shown by extensivetheoretical calculations and experiments by the inventor, that thedescribed effect is real.

The Design of the Phase-type SLM

A cloverleaf mirror design as used in prior art is possible to drive tointermediate states between on and off states. However, when theintegrated complex amplitude is plotted as a function of deflection, itis seen that it never goes to zero completely but circles around zero,therefore having a non-zero minimum reflectivity with a varying phaseangle. A thorough analysis of an image with some pixels set tointermediate states shows that the position of the edges in the finalimage are not stable through focus if the integrated phase angle of theedge pixels is not zero. In a preferred embodiment of the invention anew type of pixels with pivoting elements is used. When the elementspivots one end moves toward the light source and the other end away fromit thereby keeping the average phase close to zero. Furthermore thecloverleaf design has a problem of built-in stress created during thefabrication. This stress tends to give a partial deformation without anapplied electric field. The built-in deformation is not perfectly thesame in every pixel since it depends on imperfections during themanufacturing. In the cloverleaf design this difference from pixel topixel creates a first-order variation of reflectivity. With pixel cellsbuilt from pivoting elements the same effect occurs, but gives asecond-order effect. Therefore the uniformity is better in the projectedimage.

Image Enhancements

There is a third advantage with a pivoting design: the cloverleaf doesnot reach full extinction, but a pivoting cell can more easily be givena geometry that gives full extinction, or even goes through zero andcomes back to a small non-zero reflection, but with reversed phase. Withbetter extinction there is greater freedom to print overlappingexposures, designing for a small negative value gives better linearityclose to extinction. Printing with a weak exposure, approximately 5%, inthe dark areas, but with reversed phase can give an increased edgesharpness of 15-30% and the ability to print smaller features with agiven lens. This is an analog to so called attenuating phase-shiftingmasks that are used in the semiconductor industry. A related method ofincreasing the edge acuity is to set the pixels that are inside afeature a lower value and those near the edge a higher value. This givesa new type of image enhancement not possible with current projection ofpatterns from masks or by the use of projectors following Nelson andKück. The combination of a non-zero negative amplitude in the backgroundand an increased exposure along the edges need not conflict with thecreation of a fine address grid by driving edge pixels to intermediatevalues, since the effects are additive or at least computable. When thepixels are substantially smaller than the feature to be printed thereexists a combination of pixel values that creates all effectssimultaneously. To find them requires more computation than thegeneration of a fine address grid alone, but in some applications of theinvention the ability to print smaller features can have a high valuethat pays for the extra effort.

In the case of a continuous mirror on a viscoelastic layer there is aninherent balancing of the average phase to zero. Simulations have shownthat the driving to intermediate values for fine positioning of featureedges work also for the continuous mirror. The non-linearities aresmaller than with micromirrors. But for the method to work well theminimum feature has to be larger than with micromirrors, i.e. have alarger number of addressed pixels per resolved feature element isneeded. Consequences are a larger SLM device and that for given patternthe amount of data is larger. Therefore the micromirrors have beenchosen in a first and second embodiment.

In the invention a pixel with rotation-symmetrical deformation (at leasttwo-fold symmetry, in a preferred embodiment four-fold symmetry) is usedfor two reasons: to give a symmetrical illumination of the pupil of theprojection lens and to make the image insensitive to rotations. Thelatter is important for printing a random logic pattern on asemiconductor wafer. If there is an x-y asymmetry the transistorslaid-out along the x axis will have a different delay from those alongthe y axis and the circuit may malfunction or can only be used at alower clock-speed. The two requirements of image invariance throughfocus and symmetry between x and y makes it very important to create andmaintain symmetries in the optical system. Symmetry can be eitherinherent or it can be created by delibrate balancing of assymetricproperties, such as using multiple exposures with complementaryassymmetric properties. However, since multiple exposures lead toreduced through-put inherent symmetrical layouts are strongly favoured.

Preferred Embodiments

A first preferred embodiment is a deep-UV pattern generator forphotomasks using an SLM of 2048×512 micromirrors. The light source is anKrF excimer laser with a pulsed output at 248 nanometers, pulse lengthsoff approximately 10 ns and a repetition rate of 500 Hz. The SLM has analuminum surface that reflects more than 90% of the light. The SLM isilluminated by the laser through a beam-scrambling illuminator and thereflected light is directed to the projection lens and further to thephotosensitive surface. The incident beam from the illuminator and theexiting beam to the lens are separated by a semitransparent beamsplittermirror. Preferably the mirror is polarisation-selective and theilluminator uses polarised light, the polarisation direction of which isswitched by a quarter-wave plate in front of the SLM. For x and ysymmetry at high NA the image must be symmetrically polarised and asecond quarter-wave plate between the beamsplitter and the projectionlens creates a circularly polarised image. A simpler arrangement whenthe laser pulse energy allows it is to use a non-polarisingbeamsplitter. The quarter-wave plate after the second pass through thebeamsplitter is still advantageous, since it makes the design of thebeam-splitting coating less sensitive. The simplest arrangement of allis to use an oblique incidence at the SLM so that the beams from theilluminator and to the projection lens are geometrically separated, asin FIG. 1.

The micromirror pixels are 20×20 μm and the projection lens has areduction of 200×, making on pixel on the SLM correspond to 0.1 μm inthe image. The lens is a monochromatic DUV lens with an NA of 0.8,giving a point spread function of 0.17 μm FWHM. The minimum lines thatcan be written with good quality are 0.25 μm.

The workpiece, e.g. a photomask, is moved with aninterferometer-controlled stage under the lens and the interferometerlogic signals to the laser to produce a flash. Since the flash is only10 ns the movement of the stage is frozen during the exposure and animage of the SLM is printed, 204.8×51.2 μm large. 2 milliseconds laterthe stage has moved 51.2 μm, a new flash is shot and a new image of theSLM is printed edge to edge with the first one. Between the exposuresthe data input system has loaded a new image into the SLM, so that alarger pattern is composed of the stitched flashes. When a full columnhas been written the stage advances in the perpendicular direction and anew row is started. Any size of pattern can be written in way, althoughthe first preferred embodiment typically writes patterns that are125×125 mm To write this size of pattern takes 50 minutes plus the timefor movement between consecutive columns.

Each pixel can be controlled to 25 levels (plus zero) therebyinterpolating the pixel of 0.1 μm into 25 increments of 4 nanometerseach. The data conversion takes the geometrical specification of thepattern and translates if to a map with pixels set to on, off orintermediate reflection. The data path must supply the SLM with2048*512*500 words of data per second, in practice 524 Mbytes of pixeldata per second. In a preferred embodiment the writable area ismaximally 230×230 mm, giving up to 230/0.0512=4500 flashes maximum incolumn and the column is written in 4500/ 500=9 seconds. The amount ofpixel data needed in one column is 9×524=4800 Mb. To reduce the amountof transferred and buffered data a compressed format is used, similar tothe one in Sandstrom et al. 90, but with the difference that a pixel mapis compressed instead of segments with a length and a value. A viablealternative is to create a pixel map immediately and use commercialyavailable hardware processors for compresssion and decompression toreduce the amount of data to be transferred and buffered. Even withcompression the amount of data in a full mask makes it highlyimpractical to store pre-fractured data on disk, but the pixel data hasto be produced when it is used. An array of processors rasterise theimage in parallel into the compressed format and transfer the compresseddata to an expander circuit feeding the SLM with pixel data. In thepreferred embodiment the processors rasterise different parts of theimage and buffer the result before transmitting them to the input bufferof the expander circuit.

A Second Preferred Embodiment

In a second preferred embodiment the laser is an ArF excimer laser with193 nm wavelength and 500 Hz pulse frequency. The SLM has 3072×1024pixels of 20*20 μm and the lens has a reduction of 333× giving aprojected pixel of 0.06 μm. There are 60 intermediate values and theaddress grid is 1 nanometer. The point spread function is 0.13 μm andthe minimum line 0.2 μm. The data flow is 1572 Mbytes/s and the data inone column 230 mm long is 11.8 Gb.

A third preferred embodiment is identical with the second one exceptthat the matrix of pixels is rotated 45 degrees and the pixel grid is 84μm, giving a projected pixel spacing along x and y of 0.06 μm. The laseris an ArF excimer laser and the lens the reduction of 240. Because ofthe rotated matrix the density of pixels in the matrix is less and thedata volume is half of the previous embodiment but with the same addressresolution.

Laser Flash to Flash Variations

The excimer laser has two unwanted properties, flash-to-flash energyvariations of 5% and flash-to-flash time gitter of 100 ns. In thepreferred embodiments both are compensated in the same way. A firstexposure is made of the entire pattern with 90% power. The actual flashenergy and time position for each flash is recorded. A second exposureis made with nominally 10% exposure and with the analog modulation usedto make the second exposure 5-15% depending on the actual value of thefirst one. Likewise a deliberate time offset in the second exposure cancompensate for the time gitter of the first one. The second exposure canfully compensate the errors in the first, but will itself give newerrors of the same type. Since it is only on average 10% of the totalexposure both errors are effectively reduced by a factor of ten. Inpractice the laser has a time uncertainty that is much larger than 100ns, since the light pulse comes after a delay from the trigger pulse andthis delay varies by a couple of microseconds from one time to another.Within a short time span the delay is more stable. Therefore the delayis measured continuously and the last delay values, suitably filtered,are used to predict the next pulse delay and to position the triggerpulse.

It is possible to make corrections for stage imperfections in the sameway, namely if the stage errors are recorded and the stage is drivenwith a compensating movement in the second exposure. Any placementerrors that can be measured can in principle be corrected in this way,partially or fully. It is necessary to have a fast servo to drive thestage to the computed points during the second exposure. In prior art itis known to mount the SLM itself on a stage with small stroke and shortresponse time and use it for fine positioning of the image. Anotherequally useful scheme is to use a mirror with piezoelectric control inthe optical system between the SLM and the image surface, the choicebetween the two is made from practical considerations. It is alsopossible add a position offset to the data in an exposure field, andthereby move the image laterally.

The second exposure is preferably done with an attenuating filterbetween the laser and the SLM so that the full dynamic range of the SLMcan be used within the range 0-15% of the nominal exposure With 25intermediate levels it is possible to adjust the exposure in steps of15%*{fraction (1/25)}=0.6%.

The response varies slightly from pixel to pixel due to manufacturingimperfections and, potentially, also from ageing. The result is anunwanted inhomogeneity in the image. Where image requirements are veryhigh it may be necessary to correct every pixel by multiplication withthe pixels inverse responsivity which is stored in a lookup memory. Evenbetter is the application of a polynomial with two, three or more termsfor each pixel. This can be done in hardware in the logic that drivesthe SLM.

In a more complex preferred embodiment several corrections are combinedinto the second corrective exposure: the flash to flash variation, flashtime god gitter, and also the known differences in the response betweenthe pixels. As long as the corrections are small, i.e. a few percent ineach they will add approximately linearly, therefore the corrections canbe simply added before they are applied to the SLM. The sum ismultiplied with the value desired exposure dose in that pixel.

The use of an additional exposure could be used for many other types ofcorrection; such as light scattering and unwanted diffraction effects.

In FIG. 7 is shown intensity curves for a section on a substrate. Thedotted lines 701 indicates the intended exposure intensity, whereas theunbroken lines 702 indicates the actual exposed intensities. In FIG. 7athe section shows three exposed partial patterns stitched together, anda partial overlap between the fields are used. In the first field theintensity is slightly higher than intended, resulting in a bit widerlines than intended. In the second field the intensity is too low,resulting in too narrow lines after the etching processing. In FIG. 7b asecond exposure is made, indicated by line 703, with a pixel-to-pixelcorrection. Hereby an additional intensity is added to the intensityfrom the first exposure, resulting in a total intensity being muchcloser to the intended intensity. However, the second exposure couldinstead be a blanket exposure, i.e. not containing the pattern. This isshown in FIG. 7c, where the line 704 indicates the blanket exposure,adding an evenly distributed intensity for each field. After theprocessing the line uniformity will in this case be nearly as good as inFIG. 7b, at same time as no pattern data is needed, making the secondexposure much easier to perform.

The dose of the corrective exposure is preferably in the range 1 to 35%of the total dose, and most preferably 1 to 20%, and the SLM could beused for controlling the dose of the corrective exposure. It is alsopossible to use an attenuator, e.g. a Pockels cell, for controlling thedose of the corrective exposure. Correction for positioning errors couldbe made in the same manner. Hereby, the exposed pattern in the correctedpass is given an compensation position offset, either by changing theoptics, by moving the SLM, tilting a mirror, moving the stage or thelens, or alternatively changing the data driven to the modulator.

Alternative Illumination Sources

The excimer laser has a limited pulse repetition frequency (prf) of500-1000 Hz depending on the wavelength and type of the laser. Thisgives large fields with stitching edges in both x and y. In two otherpreferred embodiments the SLM is illuminated with a pulsed laser withmuch higher prf, e.g. a Q-switched upconverted solid state laser, andwith a continuous laser source scanned over the surface of the SLM, sothat one part of the SLM is reloaded with new data while another part isprinted. In both cases the coherence properties of the lasers aredifferent from the excimer laser and a more extensive beam-scramblingand coherence control is needed, e.g. multiple parallel light paths withdifferent pathlengths. In some implementations of the invention thelight output from a flash lamp is sufficient and can be used as thelight source. Advantages are low cost and good coherence properties.

In the preferred embodiment with scanning illumination two issues areresolved: the pulse to pulse variation in time and energy, since thescanning is done under full control preferably using an electroopticscanner such as acoustooptic or electrooptic, and many continuous laserhave less power fluctuation that pulsed lasers. Furthermore the use ofcontinuous lasers gives a different selection of wavelengths andcontinuous lasers are less dangerous to the eye than pulsed lasers. Mostimportant, however, is the possibility of reaching much higher datarates with a matrix with only a few lines since the scanning isnon-critical and can be done with 100 kHz repetition rate or higher.Scanning the illumination beam is also a way of creating a very uniformillumination, which is otherwise difficult.

In some embodiments it is possible, and feasible, to use a flash lamp asthe illumination source.

EUV

Light sources for EUV are based on radiation from a particleaccelerator, a magnetic plasma pinch machine or the heating of a smalldrop of matter to extreme temperatures with a high-power laser pulse. Ineither case the radiation is pulsed. The EUV radiation propagates onlyin vacuum and can only be focused by reflective optics. A typicalpattern generator using an SLM has a small exposure field a modestrequirement of optical power. The design of the optical system istherefore relaxed compared to that of an EUV stepper, making it possibleto use more mirrors and go to higher NA than in a stepper. It isanticipated that a high-NA lens will have a ring-shaped exposure fieldand it is fully possible to adapt the shape of the SLM to such a field.With a wavelength of 13 nm and an NA of 0.25 it is possible to exposelines that are only 25 nm wide, and, using image enhancement asdescribed below, even below 20 nm. No other known writing technology canmatch this resolution and a the same time the writing speed that is madepossible by the parallel character of an SLM.

Edge Overlap

Since a two-dimensional field is printed for each flash and the fieldsare stitched together edge to edge to edge the stitching is verycritical. A displacement of only a few nanometers of one field willcreate pattern errors along that edge that are visible and potentiallydetrimental to the function of an electronic circuit produced from themask. An effective way of reducing the unwanted stitching effects is toprint the same pattern in several passes but with a displacement of thestitching boundaries between the passes. If the pattern is printed fourtimes the stitching error will occur in four positions, but with only afourth of the magnitude. In a preferred embodiment of the currentinvention the ability to create intermediate exposures is used togetherwith an overlap band between the fields. The values are computed duringthe rasterisation, although it could also be done during the expansionof the compressed data. Edge overlap reduces the stitching errors withmuch less throughput penalty than multipass printing.

Modified Illumination

In the first preferred embodiment the illumination of the SLM is done byan excimer laser and a light scrambler such as a fly-eye lens array tocreate an illumination that resembles that from a circular self-luminoussurface in the pupil plane of the illuminator. In order to increase theresolution when printing with a specific projection system it ispossible to use a modified illumination. In the most simple cases pupilfilters are introduced in the pupil plane of the illuminator, e.g. witha quadrupole-shaped or annular transmission area. In a more complex casethe same field is printed several times. Several parameters can be madeto vary between the exposures, such as focus in the image plane,illumination pattern, data applied to the SLM and pupil filter in thepupil plane of the projection optics. In particular the synchronisedvariation of the illumination and a pupil filter can give an increasedresolution, most notably if the pupil has and a sector-shapedtransmitting area and the illumination is aligned so that thenon-diffracted light intercepts an absorbing stop near the apex of thesector.

Linearisation of the Response

For linearisation of the transfer function from data to edge placementhere are essentially three ways to go:

taking the non-linearity into account in the data conversion unit andgenerating an 8 bit (example) pixel values in the data conversion unitand use DACs with the same resolution to drive the SLM.

to generate digital values with fewer values, e.g. 5 bits or up to 32values, and translate them to an 8 bit value in a look-up table (LUT)and then feed the 8 bit values to the DACs.

to use a 5 bit value and semiconductor switches to select a DC voltagethat is generated by one or several high-resolution DACs.

In either case it is possible to measure an empirical calibrationfunction such that the response on the plate is linearised, when saidfunction being applied at respectively the data conversion unit, the LUTor in the DC voltages.

Which linearization scheme to use depends on the data rate, theprecision requirements and also on the available circuit technology thatmay change over time. At the present time the data conversion unit is abottleneck and therefore it is not a preferred solution to linearise inthe data conversion unit, neither to generate 8-bit pixel values.High-speed DACs are expensive and power-hungry and the most appropriatesolution is to generate DC voltages and use switches. It is thenpossible to use even higher resolution than 8 bits.

Description of a Preferred Pattern Generator

Referring to FIG. 6, a pattern generator comprises an SLM 601 withindividual and multi-value pixel addressing, an illumination source 602,an illumination beam scrambling device 603, an imaging optical system604, a fine positioning substrate stage 605 with an interferometerposition control system 606 and a hardware and software data handlingsystem 607 for the SLM. For proper functionality and ease of operationit also contains a surrounding climate chamber with temperature control,a substrate loading system, software for timing of stage movement andexposure laser triggering to achieve optimum pattern placement accuracyand a software user interface.

The illumination in the pattern generator is done with a KrF excimerlaser giving a 10-20 nanoseconds long light flash in the UV region at248 nanometer wavelength with a bandwidth corresponding to the naturallinewidth of an excimer laser. In order to avoid pattern distortion onthe substrate, the light from the excimer laser is uniformly distributedover the SLM surface and the light has a short enough coherence lengthnot to produce laser speckle on the substrate. A beam scrambler is usedto achieve these two aims. It divides the beam from the excimer laser inseveral beam paths with different path length and then add them togetherin order to reduce the spatial coherence length. The beam scrambler alsohas a beam homogenizer consisting of a lens system containing a set offly-eye lenses that distributes the light from each point in the laserbeam from the excimer laser uniformly over the SLM surface giving a“top-hat” light distribution.

The light from the SLM is relayed and imaged down to the substrate onthe substrate stage. This is done using a Schlieren optical systemdescribed by Kück. A lens l₁ with the focal width f₁ is placed at thedistance f₁ from the SLM. Another lens l₂ with the focal length f₂ isplaced at the distance 2×f₁=f₂ from the SLM. The substrate is then at adistance 2×f₁+2×f₂ from the SLM. At the distance 2×f₁ from the SLM thereis an aperture 608 which size determines the numerical aperture (NA) ofthe system and thereby the minimum pattern feature size that can bewritten on the substrate. In order to correct for imperfections in theoptical system and the substrate flatness there is also a focal systemthat dynamically positions the lens l₂ in the z direction with aposition span of 50 micrometers to achieve optimal focal properties. Thelens system is also wavelength corrected for the illuminating wavelengthof 248 nanometers and has a bandwidth tolerance of the illuminatinglight of at least ±1 nanometer. The illuminating light is reflected intothe imaging optical system using a beamsplitter 609 that is positionedimmediately above the lens l₁. For a demagnification factor of 250 andan NA of 0.62 it is possible to expose pattern features with a size downto 0.2 micrometers with a good pattern quality. With 32 gray levels fromeach SLM-pixel the minimum gridsize is 2 nanometers.

The pattern generator has a fine positioning substrate stage with aninterferometer position control system. It consists of a moveable airbearing xy table 605 made of zerodur for minimum thermal expansion. Aservo system with an interferometer position feedback measuring system606 controls the stage positioning in each direction. In one direction,y, the servo system keeps the stage in fixed position and in the otherdirection, x, the stage moves with continuos speed. The interferometerposition measuring system is used in the x-direction to trigger theexposure laser flashes to give uniform position between each image ofthe SLM on the substrate. When a full row of SLM images are exposed onthe substrate the stage moves back to the original position in the xdirection and moves one SLM image increment in the y direction to exposeanother row of SLM images on the substrate. This procedure is repeateduntil the entire substrate is exposed.

The SLM images overlaps with a number of pixels in both the x and ydirection and the exposure data pattern is locally modified in theoverlap pixels to compensate for the increased number of exposures thatresult in such overlap areas.

Variations in pulse to pulse intensity from the excimer laser iscompensated by using two-pass exposure of the pattern where the firstpass is done using a nominal 90% intensity of the correct intensity. Inthe first pass, the actual intensity in each laser flash is measured andstored. In the second pass, the correct intensity for each SLM imageexposure is then used based on the measured intensity values from thefirst pass. In this way it is possible to reduce the influence frompulse to pulse intensity variations from the excimer laser by one orderof magnitude.

The functionality of the SLM is described extensively elsewhere in thistext. It has 2048×256 pixels with pixel size of 16 micrometers and it ispossible to address all pixels within 1 millisecond. The SLM is rigidlymounted in a fine stage. This fine stage is moveable 100 microns in thex and y directions with accuracy better than 100 nanometers between eachflash exposure. The fine positioning of the SLM is used to correct forposition inaccuracy of the substrate positioning stage to further reducepattern-stitching errors. In addition to the x-y positioning, there isalso a rotational possibility of the SLM stage in order to expose apattern on a substrate with an angle other than the one specified by thesubstrate stage coordinate system. The purpose for such rotation is tocreate the possibility of incorporating substrate alignment feasibilityfor substrates with an already existing pattern where additionalfeatures shall be added. It is possible to measure the exact position ofthe substrate on the stage after loading it using an off line opticalchannel and a CCD camera to determine the system coordinates for anumber of alignment marks existing on the substrate. During exposure,the stage position is then corrected in the x- and y-directions based onthe measured positions of the alignment marks. Rotational alignment isachieved by using the stage servo system to follow the rotatedcoordinate system and also rotating the SLM fine stage as described.

An arbitrary data pattern of an arbitrary format is transformed into acompressed rasterized pixel map with 32 (5 bit) gray levels per pixel ina pattern rasterizer 610. Since the grayscale steps of an exposed pixelis not linear in response to the voltage applied to the pixel electrode,the input data is linearized in a pixel linearizer 611 so that the 32gray levels correspond to a uniform increase in exposure dose for eachsuccessive level. This is done using 8-bit digital to analog converters(DAC) 612 where each gray level from the pixel map selects a voltagefrom the DAC's according to a previously empirically calibratedlinearization function. An additional offset in the choice of analoglevel from the DAC's is made using a lookup table where each valuecorresponds to an SLM pixel and each such value corrects for anomaliesof the corresponding pixel. The calibration values in the lookup tableare generated using an empirical calibration procedure where a series oftest patterns are sent to the SLM and the resulting exposed patterns aremeasured and used for individual pixel correction. This means that eachgray level in the pixel map selects an analog voltage generating a pixeldeformation for every corresponding SLM pixel to give the correctexposure dose.

REFERENCES

Nelson 1988: U.S. Pat. No. 5,148,157

Kück 1990: European patent EP 0 610 183

Sandstrom et al. 1990: European patent EP 0 467 076

What is claimed is:
 1. A method for creating a pattern on a workpiecewith improved pattern fidelity and dimensional precision, comprising thesteps of: providing a source for emitting electromagnetic radiation inthe wavelength range from EUV to IR, illuminating by said radiation aspatial light modulator (SLM) having multitude of modulating elements(pixels), projecting an image of the modulator on the workpiece, movingsaid workpieces and/or a projection system relative to each other,reading from an information storage device a digital description of thepattern to be written, extracting from the pattern description asequence of partial patterns, converting said partial patterns tomodulator signals, and feeding said signals to the modulator, furthercoordinating the movement of the workpiece, the feeding of the signalsto the modulator and the intensity of the radiation, so that saidpattern is stitched together from the partial images created by thesequence of partial patterns, and further exposing said pattern in atleast two separate exposures, where one of the exposure corrections isapplied for errors occurring during the other exposure, wherein the twoseparate exposures being made on the same workpiece.
 2. The methodaccording to claim 1, where corrections are applied for light scatteringand unwanted diffraction effects.
 3. The method according to claim 1,where errors occurring at an earlier exposure are measured and recorded,and corrections for them are applied during a subsequent exposure. 4.The method according to claim 1, where errors in the exposure doseoccurring at an earlier exposure are measured and recorded, andcorrections for them are applied during a subsequent exposure.
 5. Themethod according to claim 1, where errors in the exposure dose occurringat an earlier exposure are measured and recorded, and corrections forthem are applied during a subsequent exposure; and where the correctiveexposure has a lower dose than the one being corrected.
 6. The methodaccording to claim 1, where errors in the exposure dose occurring at asequence of partially overlapping earlier exposures are measured andrecorded, and the dose error for difference areas on the workpiece arecomputed.
 7. The method according to claim 1, where the correctiveexposure is a blanket exposure.
 8. The method according to claim 1,where the corrective exposure is an exposure with the same pattern, butwith compensating corrections applied.
 9. The method according to claim1, where the dose of the corrective exposure is in the range 1 to 35% ofthe total dose, and preferably in the range 1 to 20%.
 10. The methodaccording to claim 1, where the SLM is used for controlling the dose ofthe corrective exposure.
 11. The method according to claim 1, where anattenuator is used for controlling the dose of the corrective exposure.12. The method according to claim 1, where the error recorded is aposition error and a compensating position error is created during thecorrective exposure.
 13. The method according to claim 12, where thelight source is a pulsed light source and a compensating position erroris created during the corrective exposure by alteration of the timing ofthe light pulses.
 14. The method according to claim 12, where thecompensating position error is created during the corrective exposureusing a mirror driven by actuators.
 15. The method according to claim12, where the compensating position error is created during thecorrective exposure by movement of the SLM by actuators.
 16. The methodaccording to claim 12, where the compensating position error is createdduring the corrective exposure by adding a position offset to the data.17. The method according to claim 1, where the error recorded is a doseerror and the correction applied during the corrective exposure isapplied to the data.
 18. The method according to claim 1, where theerror recorded is a focus error and a compensating focus error iscreated during the corrective exposure.
 19. The method according toclaim 11, wherein the attenuator is a Pockels cell.
 20. The methodaccording to claim 7, wherein the blanket exposure does not contain thepattern.